The present invention relates to logic circuits and more specifically to time delay logic circuits and oscillators.
There are many types of prior art time delay circuits. Often, time delay circuits included an analog time delay such as a resistive-capacitive (RC) circuit. An RC circuit receives a pulse and then delays or extends that pulse according to an RC time constant of the RC circuit. FIG. 1 shows a RC time delay circuit 100. A short pulse 102 is received in the input terminal 104 through a gating circuit 106. The gating circuit applies the input pulse to the input of the RC tank circuit 108. The RC tank circuit includes resistor 109 and capacitor 110. An extended pulse 120 is output from the RC tank circuit through an output gate 130. The extended pulse is extended according to the RC time constant of the tank circuit. The RC time delay circuit 100 has several shortfalls. One shortfall is that it is extremely dependent upon temperature and the corresponding variation of the resistance and the capacitance of the resistor 109 and capacitor 110, respectively. The dependence on temperature results in an extremely variable time delay. Another Shortfall of the RC time delay is that the RC time constant is also dependent upon the applied voltage, Vcc. Because the output of the RC time delay circuit is variable, then it is not precisely predictable or reliable unless the temperature of the resistor 109 and the capacitor 110 is also known. If the temperature of the resistor 109 and the capacitor 110 is also known, then the resulting time delay can be correlated to temperature and thereby predicted. Unfortunately, monitoring the temperature of R1 and C1 requires a more complicated circuit.
A resistive circuit can also be used in a bias current controlled oscillator 200 as shown in FIG. 2. Resistor 202 and transistor M1 set up a bias current for transistors M2 and M3. Transistors M2 and M3 create reference voltages REFP and REFN, respectively. REFP and REFN are input to an oscillator 210 that includes biasing transistors M4, M5, and three oscillator stages 212, 214, 216. Additional stages could also be included but the illustration is limited to the three stages for simplicity of discussion. The first oscillator stage 212 includes transistors M6, M7. The second oscillator stage 214 includes transistors M8, M9. The third oscillator stage 216 includes transistors M10, M11. Reference voltages REFP and REFN are used to bias the three stages 212, 214, 216 of the oscillator 210. Each of the stages 212, 214, 216 are delay inverters. The input to the stages 212, 214, 216 are linked to the output of the stages to create an oscillator. The oscillation frequency of the bias current controlled oscillator 200 is dependent upon the voltage drop across the biasing resistor 202. The voltage drop across the resistor 202 varies with the temperature of the resistor 202 or the current flow through the resistor 202. The oscillation frequency of the bias current controlled oscillator 200 is also dependent on variations in the Vcc input voltage. Further, the bias current controlled oscillator 200 also sinks a static current through the resistor 202 and the first stage 212, even when the oscillator is not oscillating.
For one embodiment of the invention, a time delay circuit comprises a first transistor and a delay element. An input voltage is applied between the gate and the drain of the first transistor. An output voltage is taken at the source of the first transistor. The output voltage of the first transistor is coupled to the delay element, biasing the delay element.